The invention concerns a method for adsorptive material separations by permeation of liquids through porous adsorbers, and an installation for carrying out the method on a process scale.
Adsorptive separation is understood in the art to mean specific separation or purification of substances (components) from a liquid phase (medium), where the substances are reversibly preferentially adsorbed by a solid adsorbent. For this purpose, a medium that contains the substances to be separated or purified is supplied to the adsorbent, or forced through it, and separated by means of one or more elution liquids (eluants) that are forced through the adsorbent under pressure. Depending upon the degree of interaction between the components of the medium and the adsorbent and eluants, the individual components are retained by the adsorbent to different degrees and thus emerge from the adsorbent in a fractionated state. The substances being separated in the medium can be adsorbed either alone or together on the adsorbent. In the latter case, the medium with the substance mixture being separated is filtered through an adsorber module until the desired substance appears at the outlet of the module. With appropriate eluants, which are, for example, filtered through the module, the target substance can be eluted from other substances retained on the adsorbent, commonly referred to as stage elution. An undesired substance or contaminant can also be separated from the medium in this manner.
Interactions between solid and liquid phases, therefore, play an important role during adsorptive material separation, in which the solid phase must exhibit a high specific surface in order to achieve high effectiveness. The solid phase should, therefore, have either a limited particle size or high porosity. Since limits are imposed on the use of extremely fine solids under practical conditions, highly porous matrices are generally used as solid phases. The use of porous matrices means that the kinetics of the fundamental process of adsorption/desorption, i.e., interaction between the components of the liquid phase and those of the solid phase, are superimposed by the kinetics of mass transport both into and out of the porous matrix. Since mass transport in known matrices occurs primarily by diffusion, for example, in particulate and porous matrices, a diffusion limitation occurs that is disadvantageous to the method's effectiveness. This is because the kinetics of the overall process are determined by the kinetics of mass transport, owing to the generally low diffusion coefficients in liquid phases. Nonparticulate matrices with continuous pore structures such as in porous membranes, on the other hand, offer the possibility of largely convective mass transport under the influence of a pressure differential. This method allows for the effective elimination of the undesired diffusion limitation.
A common way of carrying out adsorptive separation is by means of adsorption membranes, or those membranes that carry functional groups, ligands or reactants on their inner and outer surfaces that are capable of interacting with at least one substance of a liquid phase with which they come into contact. The term "adsorption membrane" is to be understood as a general term for different types of adsorption membranes, such as cationic, anionic, ligand, affinity or activated membranes, which, in turn, may be classified into different types depending upon the sorts of functional groups, ligands or reactants they possess. Porous adsorption membranes are membranes whose average pore diameter lies in the microfiltration range and is between approximately 0.1 .mu.m and 15 .mu.m. The thickness of the employed porous adsorption membranes is between approximately 100 .mu.m and 500 .mu.m.
In addition to adsorption membranes, fibrous adsorbents with adsorptive properties are also known that are formed into a flat, filter-medium-like web material. See, for example, U.S. Pat. No. 4,986,909. Adsorptive groups are applied to the fibers of this web material by surface modification. In such separations with a flat adsorbent the adsorbent is traversed by the medium. Convective transport of the liquids being treated through the adsorbent then occurs. The adsorbent, which is preferably designed as a moving endless belt, is driven at an appropriate speed, and after being traversed by the medium being filtered, is either rolled up for later treatment (rinsing, desorption, regeneration) or immediately exposed to treatment media in additional steps. A shortcoming of this method is the high mechanical stress often created on the adsorber endless belt.
Adsorbers of high binding capacity required on a process scale generally do not exhibit the sort of sharp breakthrough of the target substance achieved in small units for analytical separation purposes, especially when these adsorber units are operated in parallel in order to impart the required amount of adsorbency. This means that only part of the theoretical binding capacity of the individual units can be utilized. By "target substance" is meant that component of the medium that is reversibly bound to the adsorbent. Both useful components (product) and an interfering component (contaminant) can be involved in the separation. In addition to the required over dimensioning of the adsorber module, another shortcoming of this prior art method is that the adsorber module is not fully loaded at the time of elution, which means that the attainable concentration of target substance in the eluate is lower than theoretically possible. While the large adsorber units' drawbacks can be limited by employing a series connection of at least two units, a rigid series connection of adsorber units produces no advantages in the first phase of loading, i.e., before breakthrough of the target substance from the first stage, because the second stage is traversed only by the permeate from the first stage that is already free of target substance. Additional units, therefore, serve only to produce a pressure drop of the overall installation, without otherwise serving any useful purpose.
The basic goal of the invention is therefore to devise an improved method and installation suitable for adsorptive material separation on a process scale, in order to isolate a specified amount of target substance from a liquid medium per unit time. This goal is met by the present invention, which is summarized and described in detail below.